1. Field of the Invention
The present invention relates to a switching mode power supply device, and more particularly to an auxiliary power supply device for generating a constant auxiliary power source and supplying a control section of switching mode power supply device therewith.
2. Description of the Prior Art
A switching mode power supply converts an alternating current into a direct current and the direct current is applied to a load. According to the direct current applied to the load, a control signal from a control section is applied to a switching transistor and thereby the switching mode power supply device is controlled.
Such a switching mode power supply device, as shown in FIG. 1, includes a main power supply part 1 for converting an alternating current supplied from the external into a direct current and for applying that to the load and an auxiliary power generating section 2 for generating an auxiliary power Vcc to drive main power supply part 1.
At this time, an output terminal of a bridge diode 11 of main power supply part 1 for full-wave rectifying the alternating current supplied from the external is connected to one terminal of a capacitor 12 for smoothing the output voltage from bridge diode. The other terminal of capacitor 12 is connected to a ground. At the same time, one end of capacitor 12 is connected to an end of a first primary winding L1 of a transformer 13 for exciting the voltage outputted from capacitor 12.
An output terminal of bridge diode 11 is connected to a start-up section 14 composed of a resistance 14b connected with a diode 14a in series. An output terminal of start-up section 14 is connected to a control section 15 which is driven by the output voltage from start-up section 14 and controls the switching transistor. An output terminal of control section 15 is connected to a base terminal of switching transistor 16 for switching according to the control signal from control section 15 to thereby control current passing through the first primary winding of transformer 13. A collector terminal of transistor 16 is connected to an output terminal of first primary winding L1 of transformer 13. An emitter terminal of transistor 16 is connected to a ground.
An end of a secondary winding L2 of transformer 13 is connected to an anode of a diode 17 for half-wave rectifying an excited voltage from transformer 13. A cathode of diode 17 is connected to a terminal of a capacitor 18 for smoothing the output voltage from diode 17. An output terminal of capacitor 18 is connected to the other end of secondary winding L2 of transformer 13.
An end of a second primary winding L3 of transformer 13 is connected to an anode of a diode 21 of auxiliary power generating section 2. The other end of second primary winding L3 is connected to a ground. A cathode of diode 21 is connected to a terminal of a capacitor 22. One terminal of capacitor 22 is connected to control section 15 and the other terminal of capacitor 22 is connected to a ground.
According to the conventional switching mode power supply device constructed as above, the alternating current supplied from the external is full-wave rectified by bridge diode 11 of main power supply part 1. The voltage outputted from bridge diode 11 is applied to capacitor 12 to be smoothed. The voltage from capacitor 12 is applied to first primary winding L1 of transformer 13.
The voltage from bridge diode 11 is applied to diode 14a of start-up section 14 for the half-wave rectification. The rectified voltage is applied to control section 15. Control section 15 is driven according to the output voltage of resistance 14b and outputs the control signal C for switching transistor 16. The control signal C switches transistor 16 on.
If transistor 16 is turned on, current passes through first primary winding L1 of transformer 13 and the voltage from capacitor 12 excites secondary winding L2 of transformer 13. At that time, the magnitude of excited voltage is determined by a turn ratio between first primary winding and secondary winding L2 of transformer 13.
The voltage outputted from secondary winding L2 of transformer 13 is applied to diode 17 to be rectified. The voltage from diode 17 is applied to capacitor 18 to be smoothed and thereafter is applied to the load (not shown).
The voltage from capacitor 18 is applied to control section 15 and control signal C varies according to the output voltage from capacitor 18. The varied control signal C is applied to the base terminal of transistor 16 to thereby control the switching operation. Accordingly, the current passing through first primary winding L1 of transformer 13 is controlled.
Also, when current passes through first primary winding L1 of transformer 13, second primary winding L3 of transformer 13 is excited. Then the excited voltage from transformer 13 is applied to and rectified in diode 21. The voltage outputted from diode 21 is applied to and smoothed in capacitor 22. An auxiliary power source Vcc outputted from capacitor 22 is applied to control section 15.
In such a conventional switching mode power supply device, auxiliary power source Vcc applied to control section 15 varies since the excited voltage of secondary winding L3 of transformer 13 varies depending upon the alternating current inputted to auxiliary power generating section 2. Therefore, the variable auxiliary power source Vcc causes control section 15 to malfunction so the reliability of the product is lowered. And due to the separate installation of start-up section 14, the switching mode power supply device is difficult to be economized in size and weight.